1. Field of the Invention
This invention relates to a backing pad to be used in a holding jig for holding a semiconductor wafer in the step of mirror polishing the semiconductor wafer, a method for precision surface machining the wafer-holding surface of the backing pad, and a method for polishing a semiconductor wafer by the use of the backing pad.
2. Description of the Prior Art
Generally, a semiconductor wafer is held fast in position on a polishing carrier plate for the purpose of allowing the surface thereof to be subjected to mirror polishing. In the methods which are available in this case for holding the wafer on the polishing carrier plate, the wax method which comprises applying wax to one surface of the wafer and fixing the wafer to the carrier plate by way of the medium of the wax, the waxless method which resorts to tight adhesion by virtue of vacuum, and the waxless method which comprises applying the wafer wet to the polishing carrier plate with the aid of an incompressible material made of a porous resin are in popular use.
In the wax method, the unevenness of the thickness of the adhesive layer is directly reflected in the flatness parallelism, etc. of the wafer. The adhesive layer, therefore, must be given a uniform thickness. The work of uniformizing the thickness of the adhesive layer, however, is unusually difficult and calls for skill. Recently, the trend of integrated circuit devices toward ever higher density has come to urge growing exaction of the precision of wafer. Since the application of wax is manually carried out, the uniformity and reproducibility of the thickness of the adhesive layer have their own limits. The adhesion by the use of wax inevitably necessitates the work of removal of the used wax as an after treatment and thus has constituted itself one cause for obstructing automation of the operation of the wax method.
Various studies have been made in search of waxless methods. Japanese Patent Application Disclosure SHO 61 (1986)-14,854 discloses a method which comprises setting a wafer fast in place by virtue of surface tension as with water on an elastic material of a foamed polyurethane sheet and polishing the immobilized wafer and Japanese Utility Model Application Disclosure SHO 63(1988)-10,057 discloses a method which comprises effecting the polishing of a wafer by use of a wafer-polishing jig having a template 13 containing at least one wafer-positioning hole superposed on a work-fixing mat 15 attached fast to a plate as illustrated in FIG. 6, for example.
In the method disclosed in Japanese Patent Application Disclosure SHO 61(1986)-14,854, since the foamed polyurethane sheet is a sheetlike material or a cut piece thereof, the thickness of the sheet is not accurately uniform and the surface thereof is not flat and smooth throughout the entire area thereof. When this foamed polyurethane sheet is used, therefore, the wafer cannot be mirror polished to uniform thickness. When this foamed polyurethane sheet is hydrophilic in quality, it give rise to a thin film of water and consequently suffers from a decline of adhesive force arising from surface tension at the time that the wafer is set in place thereon. When the foamed polyurethane sheet has pores of large diameters, since the adhesive force exhibited to the wafer is lowered, the disadvantage arises that the wafer being polished moves, rotates, or flies out and the abradant permeates to the reverse side of the wafer.
In the method disclosed in Japanese Utility Model Application Disclosure SHO 63(1988)-10,057, when a wafer is polished as set in place on the polishing jig having the template 13 attached fast on the work- fixing mat 15, stress is exerted on the reverse side of a wafer 14 as illustrated in FIG. 7 and the work-fixing mat 15 contiguous to the reverse side of the wafer 14 is depressed because this mat is made of an elastic material. In contrast, the work-fixing mat 15 contiguous to the template 13 is not notably depressed because no noticeable stress is exerted on the template 13. The depression formed in the work-fixing mat 15 contiguously to the peripheral region of the reverse side of the wafer 14 is smaller than the depression formed in the central part of of the reverse side of the wafer because of the stress exerted upon the central part of the reverse side of the wafer and the elastic force inherent in the work-fixing mat 15. As a result, the peripheral region of the wafer-polishing surface is bent away above the central part thereof. When the polishing is continued in this state, a polishing stock 23 of the wafer 14 illustrated in FIG. 8 (a) is polished off by a polishing pad 22. After completion of the polishing work, a polished surface 20 of the wafer forms a sagging corner 21 in the periphery thereof as illustrated in FIG. 8 (b). Thus, the polishing work fails to give a uniform thickness to the wafer throughout its area.
This invention, intended to solve the problem of the prior art described above, has as an object thereof the production of a wafer excelling in parallelism and flatness by polishing a wafer held fast in place with a backing pad exhibiting highly satisfactory adhesive force to a wafer and excelling in parallelism and flatness.